Jeb S. Fields and Damon Abdi

As one of the most iconic trees of the South, the majestic live oak is known for its year-round foliage and sprawling branches, which create an imposing yet welcoming form. Live oaks (Quercus virginiana) play a critical role ecologically, serving as shelter and nesting for a wide range of wildlife and birds. A beautiful and reliable component in the landscape, live oaks thrive in the unique Southern climate and are widely considered one of the most hurricane-resistant trees you can plant.

Considering these features, it is no surprise to see live oaks commonly planted in yards, neighborhoods or near businesses, and people routinely plan buildings, open spaces and roadways around these beautiful trees. There is a wealth of information on live oak maintenance, specifically with regards to canopy management. However, even with the interest in and importance of live oaks, there is relatively little information available regarding the best management practices of the trees’ understories — the vegetative layer between the canopy and the ground — in a landscape setting.

The upper canopy of a tree is primarily involved in photosynthesis — the process in which a plant manufactures glucose by taking in carbon dioxide and water and releasing oxygen. The reverse reaction is respiration, in which plants take in oxygen to convert glucose into energy and release carbon dioxide. Most of this respiration occurs in the roots. With how critical the root system is, it is imperative to have a proper management plan for live oak understories.

In general, oaks are considered to have some of the more sensitive root systems, so damaging live oak roots can wreak havoc on the tree. A significant portion of live oak roots exists in the upper 12 inches of the soil. This is crucial for the tree’s access to the oxygen that is vital for respiration. This is why we tend to see live oak roots emerging from the ground more than in other species. Covering the understory with dense layers of soil or synthetic barriers can pose significant harm to the live oak by inhibiting respiration by way of oxygen deprivation. Further, construction should not be done within the dripline of the tree, which is the outermost circumference of its canopy. Concrete, gravel, pavement and similar constructions will all restrict root growth, often making it necessary to cut the roots. It is best to avoid circling the entire exterior of the dripline with impermeable surfaces, as the root system typically expands two to three times outside of this area. While the tree can still survive if only a small portion of its root system is damaged, extensive damage to the system will have severe consequences on the overall health of the tree. However, the tenacity of a live oak will not allow this tree to go down without a fight if construction occurs nearby. It is common to see sidewalk sections uplifted around a live oak, as roots in search of oxygen rise to the surface.

LSU AgCenter specialists recommend better options for managing the understory of live oaks. The safest for the tree is light mulching, such as applying pine straw or allowing the fallen leaves to collect and naturally provide an organic mulch. Regardless of mulch type, too much mulch will not only restrict oxygen flow, but also promote upward root growth. Moving the root system upward in the soil profile, or worse yet, into the mulch layer, may cause significant problems. If too much of the root system migrates upward out of the native soil, the tree will not have access to the water and nutrients it needs to thrive. Even worse, the tree will lose the anchorage that keeps these trees resistant to high winds and storms. Hence, no more than 2 inches of mulch material should be applied each year, and it is important to limit mulching if the previous year’s mulch has not significantly decomposed.

Certain plants can be safely planted in the understory. Those include Asian jasmine, English ivy and other matting groundcovers with low sun requirements. Some bunching plants, such as cast iron plants or inland sea oats, also create an aesthetic grouping around the base of the tree. Turfgrasses are not recommended below the canopy of live oaks because a healthy tree will not allow enough light for the grass below to thrive. More importantly, installation of grasses is usually done via sod, which introduces more soil than is preferred to cover the root system.

Regardless of what understory management plan is selected, the ultimate goal is to create a low maintenance and healthy environment for your live oak. A good understory management plan is predicated on protecting the roots from foot traffic and other human interference, such as the use of string trimmers, allowing the roots to thrive. A healthy root system will allow your live oak to respire more efficiently and remain anchored in the landscape, keeping this iconic tree in its place for decades to come.

Jeb S. Fields is an assistant professor and ornamental horticulture specialist, and Damon Abdi is an assistant professor of landscape horticulture, both at the LSU AgCenter Hammond Research Station.

This article appears in the winter 2023 edition of Louisiana Agriculture.

Live oak trees are an iconic southern symbol creating imposing silhouettes like this one near the entrance of the LSU AgCenter Hammond Research Station. Photo by Ashley R. Edwards

Given the time and room to grow, the sprawling branches of live oak trees can grow far enough outward and downward to reach the ground. Photo by Damon Abdi

As one of the most iconic trees of the South, the majestic live oak is known for its year-round foliage and sprawling branches, which create an imposing yet welcoming form. AgCenter file photo